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Cate numere de forma 1ab se divid cu 5 si cu 4?

Răspuns :

[tex]\overline{1ab} \; \vdots \; 5 \Leftrightarrow b\in\lbrace0,5\rbrace\\ \overline{1ab} \; \vdots \; 4 \Leftrightarrow \overline{ab}\in M_4\\ \overline{1a0} \; \vdots \; 4 \Rightarrow a\in\lbrace0,2,4,6,8\rbrace\\ \overline{1a5} \; \vdots \; 4 \Rightarrow a\in \oslash\\ Asadar, \; \overline{1ab} \; \vdots \; 20 \in \lbrace{100,120,140,160,180}\rbrace[/tex]